1. Field of the Invention
The present invention generally relates to lightwave transmission systems and, more particularly, a method and apparatus for transmitting an undistorted optical return-to-zero signal using dither signals.
2. Description of the Related Art
In the propagation of optical pulses in ultra-long haul applications, such as transoceanic transmission, numerous parameters associated with such transmission limit the capacity of the optical fiber cable system through which the optical pulses are transmitted. As known in the art, optical fibers are not strictly linear, but have a small amount of nonlinearity in their transmission characteristics. This nonlinearity makes possible a pulse transmission mode in the optical fiber that is effectively immune from the aforementioned pulse degradation factors. This mode, known in the art as xe2x80x9csoliton propagationxe2x80x9d, effectively balances out the aforementioned factors provided that the pulses have a required power level. In addition, the pulses must have a pulse shape in both the time and frequency domains that is optimum for compensating dispersion at a given power level and for reducing interference and cross-talk among pulses.
In lightwave transmission systems, optical pulses are generated using optical modulators, such as Mach-Zehnder Modulators. The optical modulators are driven by a radio frequency (RF) driving signal and are biased by a DC bias signal so as to generate a stream of optical pulses for transmitting information. The optical pulses are Gaussian shaped and thus suitable for dispersion-managed soliton applications. Such a system is described in U.S. Pat. No. 5,477,375, issued Dec. 19, 1995, to Korotky et al, which is herein incorporated by reference. In such a lightwave transmission system, if the RF driving signal and/or the DC bias signal are less than optimal, then the pulse shape of the optical output pulses will be distorted, reducing or eliminating their effectiveness in ultra-long haul transmission applications.
Therefore, there exists a need in the art for a method and apparatus for reducing distortion in the optical output pulses of a lightwave transmission system.
The disadvantages associated with the prior art are overcome by a method and apparatus for transmitting a modulated optical signal comprising a continuous source of light, a first optical modulator for generating an optical pulse stream in response to a radio frequency (RF) driving signal and a DC bias signal, and a second optical modulator for modulating the optical pulse stream with an electrical data signal. A processor dithers the phase of the RF driving signal using a first dither signal and the level of the DC bias signal using a second dither signal. The processor detects the first and/or second dither signals in the modulated optical signal and adjusts the phase of the RF driving signal and/or level of the DC bias signal to null the detected first and/or second dither signals. In one embodiment of the invention, the dither signals comprise low-level audio tones.